The invention relates to a multilayer inductor which has internal conductors of a main constituent being silver at the interior of the substrate thereof composed of a constituent belonging to spinel ferrite for attenuating noise elements, and a production method thereof.
In electronic machines, element mounting densities have kept on overcrowd, and there have been actualized mutual interference between elements or problems concerned with radiation of noises. Noise controlling means of many cases are higher harmonics of used signals, and attentions are paid to controlling of the higher harmonics. As the noise controlling means, there are multilayer inductors called as beads employing ferrite magnetic materials, means which shelters areas of circuit with metal plates to prevent bad influences to other circuit blocks, or means for avoiding spreading of noises to a next step by an LC resonance circuit.
Among them, according to the filter, a noise is not controlled, but the noise element is reflected to a front step and unexpected bad influences such as oscillation are given to the circuit, but since a multilayer inductor of noise absorption type does not cause such matters, it has been broadly used as a measure against noise.
A ceramic magnetic substance enabling to be sintered at around 900xc2x0 C. and internal conductors composed of silver or its alloy are combined and baked together with the multilayer inductor so as to form coil shaped conductors at the interior of the ceramic sintered body. By shaping the internal conductors in coil, impedance may be made large, and loss of materials can be efficiently avoided, and as a result, the shape of the element can be made small sized.
Ferrite to be used to such purpose is called as spinel ferrite and it is almost composed of NiCuZnFe2O4 based component, and when using spinel ferrite, this kind of ferrite is sensitive to stress and an apparent permeability xcexc receives influence of stress and is remarkably lowered.
The multilayer inductor is baked together with silver powder for the internal conductors and ferrite powder for the substrate, and united in a one body. Since the silver is larger than the ferrite ceramic in coefficient of linear expansion, stress is, therefore, caused at the interface between silver and ferrite by baking together with, and the apparent xcexc of the ferrite is considerably lowered. Further, the interface therebetween is breakable, and the stress at the interface is released when heat-treating as soldering, so that problems occur that the apparent permeability xcexc is changed each time of the heat treatment and the characteristics are unstable.
For solving the problems, in JP-A-4-65807, a space 3 is defined as seen in FIG. 7 between the internal conductors 2 made of silver and the substrate 1 made of ferrite, thereby easing the stress exerting between the silver internal conductor 2 and the substrate 3 so as to improve the permeability xcexc. Numeral 4 designates external electrodes connected to both ends of the internal conductors.
However, as the multilayer inductor is based on a premise of the mounting by soldering, it is necessary to perform an electrolytic plating on the external electrodes 4, and a plating liquid goes into the space 3 between the internal conductor and the substrate. The plating liquid has an etching effect to ferrite and various bad influences to the ferrite substrate.
In view of the above mentioned problems, it is accordingly an object of the invention to offer multilayer inductors stable in characteristics and a production method thereof, in which a spinel ferrite is a substrate and silver or its alloy is internal conductors, thereby to ease stress between the internal conductors and the substrate.
A multilayer inductor of a first aspect of the invention carries conductors of a main constituent being silver at an interior of a substrate composed of a constituent belonging to spinel ferrite, said internal conductors being drawn outside of the substrate, and the drawn portions are provided with external electrodes, and is characterized in that manganese and bismuth are contained in the internal conductors, and the manganese and bismuth contents at an interface between the internal conductors and the substrate are more than those of other ranges.
In the invention, manganese and bismuth are interposed between the internal conductors and spinel ferrite as the substrate for moderating stress. This fact may be explained as follows. A manganese element is in general included in a ferrite crystal lattice as known in MnZn ferrite. It is known that MnZn ferrite is less effected with stress in comparison with NiCuZn ferrite used in the invention. However, MnZn ferrite is higher in a sintering temperature than a melting point of silver as the internal conductor used in the invention, and it is assumed that a portion is made where the sintering is not partially progressed with only addition of manganese.
On the other hand, bismuth is known to lower the temperature of the sintered. ferrite. In the invention, it is possible to provide such a multilayer inductor where the sintering does not progress with only addition of manganese, but by adding bismuth, a sintering at lower temperature is available, enabling to use silver as the internal conductor. Thus, the closeness of the inductor advances, so that stress at the interface is relieved.
A method of producing the multilayer inductor of a second aspect of the invention, is characterized in that MnO2 of 0.02 to 0.1 wt % and Bi2O3 of 0.5 to 1.2 wt % are added to a paste of a main constituent being silver to be used to the internal conductors, and the paste is baked together with spinel ferrite material.
In the production method of the invention, if an addition content of MnO2 is less than 0.02 wt %, impedance before the plating is lowered, and as a result, the changing rate of impedance before and after the plating is large, and the changing rate of a soldering heat resistance test is also large. On the other hand, if being higher than 0.1 wt %, acquisition impedance is considerably lowered. A preferable addition amount of MnO2 is 0.05 wt % to 0.07 wt %.
On the other hand, if the addition amount of Bi2O3 is less than 0.5 wt %, the impedance before the plating is low and the changing rate of the impedance before and after the plating is large, and the changing rate of the impedance at the soldering heat resistance test is also large. If the addition amount of Bi2O3 is more than 1.2 wt %, the acquisition impedance is also remarkably lower. The more preferable addition amount of Bi2O3 is 0.8 wt % to 1.0 wt %.